1. Field of the Invention
The present invention relates to a method of controlling the transmission/reception of data in a local communication network, comprising a plurality of transmitter/receiver stations, each of which is connected to a transmission medium for the transport of data, said stations forming a logic communication loop, each station having its own address and comprising an address indicator, said method comprising the following steps:
(a) the location of the right to transmit to the station whose address corresponds to the address indicated by the address indicator; PA0 (b) transmission of a single message by the station having the right to transmit and, if this station has no information to be transmitted when it is allocated the right to transmit, the transmission of a presence message by said station; PA0 (c) advancing of said address indicators by one position by an end-of-message indication of the message transmitted by the station having the right to transmit; PA0 (d) the resetting of said address indicators when each of said stations has had the right to transmit. PA0 detection by a predetermined station, referred to as the pilot station, of the absence of transmission of a message of any type by the station having the right to transmit; PA0 transmission by said pilot station of a substitution message after the detection of said absence of transmission. PA0 (a) waiting by said local station for a resetting message to be transmitted by the pilot station; PA0 (b) reception of the resetting message; PA0 (c) waiting for the indication of its own address by its address indicator; PA0 (d) when its own address is indicated by its address indicator, verification of the transmission by the pilot station of a substitution message and upon reception of said substitution message, the termination of the observation cycle; PA0 (e) when said substitution message is not received when its own address is indicated by its address indicator, activating an error signal.
2. Description of the Prior Art
A method of this kind is used in the data transmission system described in U.S. Pat. No. 4,017,841. The system described in said U.S. patent comprises a plurality of stations which are connected to a transmission medium for the transport of data. The address indicator of each station is formed by an allocation counter and its address is formed by a number. The allocation counters are all synchronized and each station is allocated the right to transmit when its allocation counter indicates its number. If a station does not have any message to be transmitted, it transmits a presence message which is in this case simply the end-of-message indication whereby each information message transmitted is normally terminated. The end-of-message indication in its turn increments the allocation counters of the various stations. When each station has had the right to transmit, all allocation counters are reset to zero, so that a new cycle is started during which each station will again be allocated the right to transmit.
A great future is predicted for local communication networks because they concern industrial or private applications, teledistribution of messages, texts, sound or images; for these applications, the local stations may be television receivers, telephones, industrial machines, computers or peripheral equipment; the transmission medium, also referred to as channel, is physically formed by a network of coaxial cables or optical fibers or any other high-frequency data transport means.
The drawback of the control method used in the data transmission system according to U.S. Pat. No. 4,017,841 consists in the fact that if a station which has the right to transmit remains silent, the entire system stops because it remains locked to this silent station. Because this station is silent, it does not transmit the end-of-message indication, so that the allocation counters are not incremented and the right to transmit is not allocated to the next station of the logic communication loop; such a silence is a quite normal situation, for example, in office applications where a station may be disconnected in the absence of the user.